You may be able to pass on the virus, even if you don't feel sick yet.

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One of the most worrisome things about the flu—aside from the gnarly symptoms—is that you can walk around with the virus without even knowing you're infected. Unfortunately, that also means you can spread influenza to plenty of other people during this time—again, without having a clue that you even have the flu in the first place.

It's known as the flu's incubation period, and "it's a big reason why the flu is so easily spread," Kathryn Boling, MD, a primary care physician at Baltimore's Mercy Medical Center, tells Health. But while you've likely heard all about symptoms of the flu (fever, cough, fatigue, etc.), best prevention tactics (the flu shot), and even any possible remedies (Tamiflu), you might be a little less sure about what the incubation period of the flu is, what that means, and if there's any way to know if you're in it. Here's what experts want you to know.

First, a refresher on the flu

To understand the flu's incubation period, it's first a good idea to go over the virus itself. Influenza, aka the flu, is a contagious respiratory infection caused by several flu viruses that infect the nose, throat, and lungs, according to the National Institute of Allergy and Infectious Diseases (NIAID).

Each year, the flu makes millions of people sick and causes thousands of hospitalizations and flu-related deaths. During the 2019-2020 flu season, for example, the Centers for Disease Control and Prevention (CDC) estimates there were up to 56 million illnesses, 740,000 hospitalizations, and 62,000 deaths from the flu in the US.

Anyone can get the flu and spread it to others—even healthy people, the NIAID says. The virus can be especially risky for certain groups, including the very young, elderly, pregnant women, and people with certain medical conditions, which is why it's so scary that you can have influenza and not know it.

People who get infected with the flu may develop symptoms like fever, chills, muscle aches, coughing, congestion, headache, and fatigue for about a week. Most flu patients get better within two weeks, but some people can develop serious complications, like pneumonia, NIAID says.

OK, so what is an incubation period?

It's important to point out that an incubation period isn't unique to the flu. COVID-19, for example, also has an incubation period, as do plenty of other infectious diseases.

"An incubation period is basically the timeframe from when you are exposed to some kind of pathogen to when you start showing signs or symptoms of that that disease process," Erik Blutinger, MD, an emergency medicine physician at Mount Sinai Queens, tells Health. So, if you pick up the flu from a friend who is sick, the time between when you hung out and when you start showing symptoms is the incubation period.

Your body begins mounting a response to the virus as soon as you're exposed to it, through something called your innate immune system, or the one you were born with that helps keep harmful materials from entering your body. Once the virus gets past the innate immune system—after the virus has had time to multiply in your body—your more targeted adaptive immune system kicks in, and you symptoms ramp up.

How long is the incubation period of the flu?

There's no exact time frame that's the same for every person with the flu but, in general, the CDC says that the incubation period of the flu can range between one and four days, with the average length of time being two days.

People with the flu are the most contagious the first three to four days after they were infected, per the CDC, which can coincide with the incubation period. Most healthy adults may be able to infect others starting the day before they show symptoms, and continue to be infectious up to seven days after they become sick, the CDC adds.

"People have the flu and are out and about doing their regular stuff, coming into contact with others before they get sick," Dr. Boling says. "You can infect plenty of people during that time."

Will you know if you're in the incubation period for the flu?

Nope. "Generally, the flu is only recognized once symptoms develop," Dr. Blutinger says. "The incubation period isn't easily detected."

Hindsight really is everything, Dr. Boling says: Once you develop symptoms of the flu, you have at least some clue of when your incubation period was.

The only way to really have a tip-off of when your incubation period is when you're actually living through it is if you know you've been exposed to someone with the flu, Dr. Boling says. But, unfortunately, most people are infected without knowing it.

The bright side of all of this, if there even is such a thing, is that the proven methods to prevent the spread of COVID-19 also work to lower your risk of both contracting and passing on the flu. So, if you happen to catch the flu, wearing a mask while you're out and about and practicing social distancing should go a long way toward making sure your incubation period doesn't harm anyone else.

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